Recently, cordless or portable applications have been increasingly realized also for equipment such as electric tools, and there is an increasing demand for portable power source systems. While electric tools such as an electric screwdriver are widely used for business purposes in construction sites and the like since they can dramatically improve working efficiency, they are also sold at places like home improvement stores and coming into use for the purpose of do-it-yourself and the like.
Although nickel-cadmium batteries, nickel-metal hydride batteries and the like are mainly used as the power sources for equipment such as electric tools that have been adapted to use in cordless applications, these batteries have a heavy weight, and therefore the use of lithium ion secondary batteries, which are lightweight, has been attempted lately. In addition to being lightweight, lithium ion secondary batteries have a discharge voltage per battery that is about three times as high as that of nickel-cadmium batteries or nickel-metal hydride batteries, and thus are advantageous in applications requiring high output.
Lithium ion secondary batteries, however, may not be able to maintain the battery performance such as the cycle life if the charge/discharge conditions are not controlled appropriately. When they are discharged under a special condition, including, for example, a case in which the positive terminal and the negative terminal are short-circuited, the battery performance may be significantly impaired.
In portable power source systems used in electric tools and the like, as the connecting mechanism between a battery pack and a mounting portion for mounting that battery pack, those of the inserting type or the sliding type have been commonly used. However, a pair of connection terminals included in the battery pack is often exposed to the outside. In addition, in the case of electric tools used for business purposes, it is often the case that a plurality of battery packs are prepared in advance and these are replaced for use, in order to improve the working efficiency. In such a case, it is conceivable that a battery pack having terminals that are exposed to the outside thereof are placed in a tool box. In the tool box, metallic tools or parts such as nails or screws may be placed, or the tool box itself may be made of metal. Due to such external factors, there is the possibility that short circuit may occur between the terminals of the battery pack.
when the terminals of the battery pack are exposed to the outside, there is also the possibility that charging may be performed with a power source other than a dedicated charger. Also in such a case, the charge conditions cannot be controlled appropriately, so that there is the possibility that the battery performance may be significantly impaired.
There is a technique that proposes to cover the terminals with a cover in a portable power source system, except for when the battery pack is connected to a dedicated charger or equipment, from the viewpoint of obviating the problems as described above (see Japanese Unexamined Patent Publication No. Hei 9-320554). In this case, short circuit will not occur unless lead wires or the like are intentionally connected to the terminals; however, short circuit or the like cannot be sufficiently prevented since the terminals are exposed in a state where the cover is released.
There is also proposed a technique by which the terminals are provided inside the battery pack, rather than being exposed to the outside, thereby preventing short circuit (see Japanese Unexamined Patent Publication No. 2001-57204). However, by simply inserting off-specification lead wires or the like into external terminal inserting portions, the lead wires or the like can be connected to the terminals, so that the possibility of charging/discharging of the battery under inappropriate conditions has not been totally eliminated.